Information can be obtained, transmitted, processed and represented in the form of digital or analog signals. Since the individual processing steps for the two types of signal require different types of engineering effort, e.g., circuit design, and cannot always be carried out with the same accuracy, it can be advantageous to convert this information from one form of siganl into the other before carrying out various processing steps.
In one conventional technique, digital signals are converted into analog signals by feeding pulse-shaped voltages which correspond to the digital signals and which appear successively in time to a capacitor, where the voltages are summed to produce a voltage corresponding to the analog signal. To be useful, this technique must normally satisfy three requirements: the setting-up or generating time for the analog signal should be short, the ripple in the analog signal caused by the summation of the individual voltage values should be as small as possible, and the analog signal ripple should be affected as little as possible by those characteristics of the electronic components which are temperature dependent.
Heretofore, two different methods, which can be simply implemented with commercially available and relatively cheap components, have been used to carry out the aforementioned type of digital to analog conversion. These two methods convert, respectively, pulses corresponding to the digital signals and having a constant repetition frequency and variable pulse width and pulses having a constant pulse width and variable repetition frequency. Practical experience has shown, and calculations have confirmed, that neither of these two methods optimally meets the three abovementioned requirements.
In the conversion of pulses having a constant repetition frequency and variable width, the temperature-dependent characteristics of the electronic components have only little influence on the analog value, but the generation time is too long for most applications. Further, the ripple depends on the ratio of the pulse duration to the duration of the conversion period. The ripple reaches its highest value when the duration of the pulse is approximately half as long as the duration of the conversion period and almost reaches a zero value only when the ratio approaches zero or 1.
In the conversion of pulses having a constant width and variable repetition frequency, the generating time is very short and the ripple decreases with an increase in the ratio of pulse width to pulse spacing, practically reaching a zero value when the pulse width approximates the pulse spacing, but the temperature-dependent characteristics of the electronic components have a relatively large influence on the analog value and this influence increases with an increase in the repetition frequency and in the number of pulses in one conversion period.